Optical communication systems are systems in which data is transmitted as light over optical fibers. It is widely recognized that single-mode fiber capacity may soon approach a limit imposed by fiber nonlinearity, called the nonlinear Shannon limit. Since time-, wavelength-, and polarization-division multiplexing have all been utilized already, recently the research community has started to explore the spatial dimension, utilizing so-called space-division multiplexing (SDM), in order to further increase the fiber capacity to meet ever-growing capacity demand.
A mode of an optical fiber is a self-consistent, transverse intensity profile that maintains its shape as the light propagates down the fiber. An optical fiber has only a finite number of guided propagation modes, the intensity distributions of which have a finite extent around the fiber core. The number of guided modes, their transverse amplitude profiles, and their propagation constants depend on the details of the fiber structure (i.e. core and cladding diameters and core and cladding refractive indices) and on the optical frequency. A single-mode fiber supports only a single guided mode per polarization direction, the lowest-order mode (LP01), which has an intensity profile similar to that of a Gaussian beam. SDM can be realized by several possible methods. One of the methods for space-division multiplexing includes transmitting multiple independent signals over different spatial modes of a multi-mode or few-mode fiber. Other methods for space-division multiplexing include transmitting multiple signals over multiple single-mode cores in an “uncoupled” (actually very weakly coupled) multi-core fiber or over super-modes in a coupled multi-core fiber.
However, for an SDM system based on few-mode fiber or coupled-multi-core fiber, there usually exists large inter-modal dispersion between the orthogonal spatial modes (except for the degenerate modes). Also, non-ideal fiber manufacturing and the cabling process, as well as accidental fiber bending, may result in mode coupling when the signal propagates down the fiber.
In order to properly de-multiplex the signal at the receiver, a multi-input-multi-output (MIMO) adaptive filter would be required. For a typical long-haul transmission system, prohibitively long tap lengths would be required (i.e., tens of thousands of taps typically would be required for just a three-mode fiber). Moreover, the signals transmitted on different spatial modes in a multi-mode fiber (or a coupled multi-core fiber) may experience different amounts of loss. Such mode-dependent loss will also limit the overall fiber capacity. For an SDM system using “uncoupled” multi-core fibers, MIMO processing is not commonly used for spatial mode separation at the receiver. However, a mode-dependent crosstalk still may be detected in the SDM system using “uncoupled” multi-core fibers. For example, for a regular 7-core fiber, crosstalk in the center core will be significantly higher than in the 6 other cores. For the case of 7-core fiber, the center core has worse transmission performance, which limits the fiber capacity.